Method for eliminating multiple-access interference and a mobile station

ABSTRACT

A mobile station, and a method for eliminating multiple-access interference in a CDMA cellular radio system having a plurality of cells each having at least one base station communicating with mobile stations residing in the respective cell and informing the respective mobile stations of at least one spreading code used in a neighboring cell, the mobile stations measuring the code phase and power level of a channel of the neighboring cell using the known spreading code. To reduce the effect of multiple-access interference, the signal received at the mobile station is utilized in the detection of the desired signal by using the code phase and power level of the measured channel using the known spreading code of the neighboring cell.

This application claims benefit of international application PCT F19400478filed Oct. 21, 1994.

This application claims benefit of international application PCT F19400478filed Oct. 21, 1994.

BACKGROUND OF THE INVENTION

The invention relates to a method for eliminating multiple-accessinterference in a CDMA cellular radio system having cells eachcomprising at least one base station communicating with mobile stationsresiding in the cell and informing the mobile stations of at least onespreading code used in a neighbouring cell, the mobile stationsmeasuring the code phase and power level of a channel of theneighbouring cell using the known spreading code.

CDMA is a multiple access method based on the spread spectrum technique,and it has been applied recently in cellular radio systems together withthe earlier developed FDMA and TDMA techniques. CDMA has severaladvantages over the earlier developed techniques, such as higherspectral efficiency and simple frequency planning.

In CDMA, the narrow-band data signal of the user is multiplied to arelatively broad band with a spreading code having a considerablybroader band than the data signal. Known test systems use bandwidthssuch as 1.25 MHz, 10 MHz and 25 MHz. The multiplication spreads the datasignal over the entire available band. All users transmit on the samefrequency band simultaneously. A spreading code is assigned to eachconnection between a base station and a mobile station, and the signalsof different users can be distinguished from each other in the receiverson the basis of the spreading code of each user.

Correlators provided in the receivers are synchronized with the desiredsignal, which is recognized on the basis of the spreading code. In thereceiver, the data signal is restored to the original band bymultiplying it again with the same spreading code as at the transmissionstage. In an ideal case, signals multiplied with some other spreadingcode do not correlate and are not restored to the narrow band. From theviewpoint of the desired signal, they thus appear as noise. One attemptsto select the spreading codes of the system in such a way that they areorthogonal with respect to each other, i.e. do not correlate with eachother.

In a typical cellular radio environment, signals between a mobilestation and a base station propagate over several paths between atransmitter and a receiver. This multipath propagation is mainly due toreflections of the signal from the surrounding surfaces. Signals thathave propagated over different paths arrive at the receiver at differenttimes due to their different propagation time delays. CDMA differs fromconventional FDMA and TDMA in that the multipath propagation can beutilized in signal reception. A so-called rake receiver comprising oneor more rake branches or correlators is a widely used receiver solutionin CDMA. Each correlator is an independent receiver unit, the functionof which is to assemble and demodulate one received signal component.The implementation of a rake branch is described more closely in ModernCommunications and Spread Spectrum, Chapter 12, G. Cooper, C. McGillem,McGraw-Hill, New York 1986. A CDMA receiver typically comprises aseparate impulse response measuring equipment, the function of which isto search out different signal components transmitted with a desiredspreading code, and detect the phases of the signal components. Eachrake branch or correlator can be controlled so that it will besynchronized with a signal component propagated over a different path.In a conventional CDMA receiver, the signals of the correlators arecombined in an advantageous way, thus obtaining a signal of highquality. The signal components received by the correlators may have beentransmitted from one base station, or in the case of macrodiversity,from a plurality of base stations.

Generally speaking, the spreading codes are not orthogonal at all valuesof the delay. Accordingly, signals delayed in different ways causeinterference in the signal detection. Such interference caused bydifferent users to one another is called multiple-access interference.

Each base station transmits to all of the mobile stations residingwithin its area on the same frequency band. The same frequency band istypically also used in adjacent cells. To minimize multiple-accessinterference, the spreading codes used by each particular base stationwithin a cell are attempted to be selected so that they are orthogonalwith respect to each other. Power control is also used to eliminate theeffect of multiple-access interference within a cell.

In the CDMA cellular radio system, it is possible to use a channelcalled a pilot channel. The pilot channel is a data-unmodulated signaltransmitted with a certain spreading code. Thus it contains no data. Thepilot channel is transmitted on the same frequency band on which theactual traffic channels are located; the pilot channel can bedistinguished from its traffic channels only on the basis of thespreading code. The pilot channel is used e.g. in power measurements andin generating a coherent phase reference. A base station may also informthe mobile stations moving within its area about the spreading codes ofthe pilot channels of the base stations of neighboring cells. In thisway, the respective mobile stations are able to identify thetransmissions of the neighbouring cells. This informing can be utilizedin the handover procedure.

When a mobile station approaches the edge of the coverage area of itspresently dedicated base station, a signal from a neighboring basestation, which is within the same frequency range as the transmission ofthe presently dedicated base station, begins to appear as anincreasingly strong interference in the receiver of the mobile station.This interference is particularly disadvantageous, as the spreadingcodes used in the neighboring cells are not necessarily fully orthogonalwith the codes used in the mobile station's own present cell. Moreover,the power control of the base station of the neighboring cell, if thereis any in use, does not take the adjacent cells into account.

SUMMARY OF THE INVENTION

The object of the persent invention is to reduce the effect ofmultiple-access interference in a situation where a desired signal froma presently dedicated base station is at minimum and interference fromat least one neighbouring cells is at maximum, i.e. within the boundaryarea of at least two cells.

This is achieved by means of a solution according to the methoddescribed in the foregoing background section which is characterized inthat the code phase and power level of the measured channel using theknown spreading code of the neighboring cell are utilized in thedetection of the desired signal from the received signal at the mobilestation.

The invention also relates to a mobile station intended for use in aCDMA cellular radio system having cells each comprising at least onebase station communicating with mobile stations residing in the cell,the mobile station having means for measuring the code phase and powerlevel of a channel using a known spreading code of a base station in aneighboring cell. The mobile station according to the invention ischaracterized in that the mobile station comprises means for detecting adesired signal from a received signal by utilizing the code phase andpower level of the measured channel using the known spreading code ofthe neighboring cell.

As already mentioned, a CDMA receiver is typically implemented by therake technique, in which the receiver comprises several correlatorbranches which can synchronize with signal components propagated indifferent ways. In the solution according to the invention, however,correlator branches are utilized in eliminating interference. All of thecorrelators are not synchronized with a signal propagated over severalpaths from the base station to the antenna of the mobile station andusing the base station's own spreading code; instead, one or more of thecorrelators are synchronized with a signal transmitted from the basestation of a neighboring cell and using a known spreading code assignedto another connection. In this way, the receiver is able to utilizeinformation about the code phases and power levels of the signals in thedetection of the desired signal.

The method according to the invention is particularly advantageous whenthe CDMA system uses complementary codes as spreading codes. In such asystem, there occurs no intra-cell multiple-access interference, as allof the spreading codes used within the cell are orthogonal with respectto each other. All of the interference thus originates from theneighbouring cells.

A further advantage of the method according to the invention is that thebase station can reduce the transmission power of a signal to a mobilestation without deteriorating the quality of the connection, as theinvention allows the quality of the signal of the mobile station to beimproved in areas where the mobile station is at the greatest distancefrom the base station. In this way, the adjacent cells are interferedwith to a lesser extent than what has been possible previously.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described more closely withreference to the examples of the attached drawings, in which

FIG. 1 shows a diagram illustrating a portion of a cellular networkwhere the method according to the invention is applicable;

FIG. 2 illustrates a situation where a mobile station is located closeto the edge of a cell; and

FIG. 3 shows an example of an implementation of the mobile stationaccording to the invention.

DETAILED DESCRIPTION

FIG. 1 is a diagram illustrating a portion of a CDMA cellular radiosystem. Two base stations BTS1 and BTS2 are connected to a base stationcontroller BSC by digital transmission links 11. The coverage area ofone base station BTS typically forms one radio cell. The base stationcontroller communicates with the other parts of the cellular network andwith a fixed network.

FIG. 2 shows a situation where a mobile station 24 in communication withthe base station 20 servicing a cell 21 has moved to the boundary areabetween cells 21 and 23. Cell 23 is serviced by the base station 22. Inthe following, one preferred embodiment of the invention will bedescribed by way of example with reference to the situation shown inFIG. 2, where there is a single neighboring cell, assuming that thesystem utilizes a pilot channel, without, however, limiting theinvention to this embodiment.

Upon approaching the edge of cell 23, the mobile station experiences anincrease in the interference level of the connection when it isreceiving a signal from its presently dedicated base station. Thismultiple-access interference is caused by the base station 22 servicingcell 23, the transmissions of the base station 22 taking place at thesame frequency as in the mobile station's own present cell 21. Thespreading codes used in the neighboring cells are not fully orthogonalwith respect to each other, which results in increased level ofinterference in a signal received at the receiver of the mobile stationwith its own spreading code. This phenomenon is particularly apparentwhen complementary codes are used, in which case there occurs nointra-cell multiple-access interference. Upon approaching the edge ofthe cell, a previously noiseless signal deteriorates gradually.

The base station 20 has informed the mobile station 24 about thespreading codes of the pilot channels of some of the neighboring basestations on one of the control channels assigned to the connectionbetween the base station 20 and the mobile station 24. The searcherbranch of the CDMA receiver of the mobile station 24 searches both formulti-path-propagated signal components transmitted from its presentlydedicated base station 20 with the mobile station's own spreading codeand signals provided with the pilot spreading codes of the neighboringbase stations.

When the mobile station 34 approaches the edge of cell 23, its searcherbranch detects the transmission on the pilot channel of the neighboringbase station 22, is synchronized with it and measures the power level ofthe transmission. The mobile station 24 reports these data to itspresently dedicated base station 20 in order that the handover to theneighboring base station 22 can be started in due course when thestrength of the pilot signal of the base station 22 as detected by themobile station 24 has become high enough.

The mobile station is also able to request from its presently dedicatedbase station information about the spreading codes of the trafficchannels of the neighboring base station 22 for the purpose ofinterference elimination. The present dedicated base station 20 forwardsthe request to the base station controller BSC, which transmits therequested data via the base station 20 to the mobile station 24.According to the needs and capacity, the mobile station can be informedof one or several spreading codes.

If the base stations employ connection-specific power control, theytransmit at unequal transmission powers on different connections. Thegreater the distance from the mobile station to a base station, thehigher the power level on which a base station is able to transmit asignal addressed to this particular mobile station. Connection-specificpower control is preferred when codes used within a cell do notcorrelate with each other, such as when orthogonal codes are used. Inaddition, connection-specific power control is to be preferred when thesystem employs interference elimination, as methods for interferenceelimination are the more efficient, the clearer the differences betweenthe power levels of the interfering signals. In such a case, the basestation controller may inform the mobile station only of spreading codesused on connections over which the neighboring base station uses a hightransmission power level, that is, on connections that interfere withthe mobile station most severely.

After having been informed of the spreading codes of the trafficchannels, the searcher branch of the mobile station is able to detectthe code phase of the desired traffic channels and to synchronize withthem and, if required, to measure the power level of the transmission.If the connection-specific power control is not in use, it is notnecessary to measure the power level, as the power level of the trafficchannel can be estimated from the power of the pilot channel alreadymeasured.

The mobile station is thus able to receive both the desired signal fromits presently dedicated base station and a signal from a neighboringbase station the spreading code of which it knows. Interferenceelimination may be an iterative process, so that the number of the rakecorrelators of the CDMA receiver does not limit the number of signals tobe eliminated.

According to a preferred embodiment of the invention, the mobile stationcalculates the cross-correlation between the received desired signal andinterfering signals received with the known spreading codes. The codephases of the signals have to be taken into account in the calculation,as the correlation of the spreading codes with respect to each otherdepends on the phase difference between them. In addition to this, theresult has to be weighted by the received power level of each signal tofind out the actual interference caused by each signal in the desiredsignal.

After having calculated the interference caused by the received signalsof the neighboring cell, the mobile station can eliminate the effect ofthis interference from the desired signal. To eliminate theinterference, each detected interfering signal is again multiplied withthe spreading code and subtracted from the received signal, which thencan be re-detected. The re-detected signal no longer contains theinterfering signals eliminated as described above.

In addition to the above-described embodiment concerned with theelimination of interference, the invention can also be applied byperforming detection in the receiver by taking all of the receivedsignals into account at the same time. The deterministic properties ofthe signals interfering with the desired signal, such as their powerlevel and code phase, are utilized in the detection.

FIG. 3 illustrates the configuration of a mobile station MS according tothe invention. In the receiving direction the mobile station comprisesan antenna 30 which forwards the received signal via radio-frequencysections 31 to an A D converter 32. The converted signal is applied toRAKE correlators 34a to 34d, each one of which has synchronized with adifferent signal component originating from one or another of the basestations. In addition, the A D-converted signal is applied to a searchercorrelator 33, the function of which is to search for signal componentstransmitted with a desired spreading code by measuring the impulseresponse of the channel. From the correlators, the signal is applied tomeans 35 which preferably combine the received signal components anddetect the signal. From the combiner the signal is applied to a channeldecoder 36 and further via a speech decoder 37 to a loudspeaker 38.

In the transmission direction, a signal from a microphone 39 is appliedvia a speech coder 40 and a channel coder 41 to spreading coding 42,from which the signal is applied via the RF sections 31 to the antenna30. The mobile station further comprises means 43 controlling all of theabove-mentioned blocks. The means 43 of the mobile station can storeinformation about the spreading codes used on the connections ofneighboring base stations possibly interfering with the desired signal.This information can be forwarded to the correlators 33, 34a to 34d ofthe mobile station, which are able to detect the code phase of a signalusing the known spreading code of a base station of the neighboring celland measure its power level. In the detection of the desired signal, themeans 35 can utilize the code phases and power levels of the signals ofthe neighboring base station received by the correlators 34a to 34d.

According to a preferred embodiment of the invention, the means 43 ofthe mobile station are able to estimate the interference caused by themeasured channel of the neighboring cell by calculating across-correlation between the desired signal transmitted from thepresently dedicated base station and the measured signal from theneighboring base station on the basis of the code phases of the signalsand weighting the cross-correlation by the power levels. The mobilestation further comprises means 43 for eliminating the calculatedinterference from the desired signal.

In addition to the above alternatives, the mobile station can also beimplemented so that all of the received signals are taken into accountat the same time in the detection.

Even though the invention has been described above with reference to theexample shown in the attached drawings, it is obvious that the inventionis not limited to this example, but it may be modified in various wayswithin the inventive idea claimed in the attached claims.

We claims:
 1. A method for eliminating multiple-access interference inCDMA cellular radio system having a plurality of cells each comprisingat least one base station arranged for communicating by CDMA with mobilestations present in a geographic area covered by the respective cell,comprising the steps of:at least one said base station of one said cell,being a presently dedicated base station relative to at least arespective one of said mobile stations and being bordered by at leastanother, neighboring said cell having a respective neighboring basestation, informing said mobile station of at least one CDMA spreadingcode used in said one cell and at least one CDMA spreading code used inat least one said neighboring cell; said one base station and at leastone said neighboring base station respectively transmitting componentsof a desired signal and a signal to be measured, on respective channels,using respective CDMA spreading codes; said mobile station measuring thecode phase and power level of a respective channel of each of said onecell and said at least one neighboring cell, using respective CDMAspreading codes as made known to said mobile station by said practicingof said informing step; and said mobile station detecting, from signalcomponents thereby received, said desired signal, by eliminatinginterference from said signal components as received by said mobilestation, based on said measuring.
 2. The method of claim 1, wherein saiddetecting by eliminating comprises:said mobile station estimatinginterference caused by at least one said respective channel of said atleast one neighboring cell using a DMS spreading code for the respectiveneighboring cell as made known to said mobile station by said practicingof said informing step, by:calculating a cross-correlation betweencomponents of said desired signal as detected from respectivetransmissions of said one base station and said at least one neighboringbase station, and weighting said cross-correlation with respective powerlevels obtained in said measuring step; and said mobile stationsubtracting interference thereby estimated, from the signal componentsreceived, to obtain the desired signal.
 3. The method of claim 1,wherein:said at least one CDMA spreading code used in said at least onesaid neighboring cell, of which said base station informs said mobilestation in said informing step comprises a spreading code used on atraffic channel of a respective neighboring cell.
 4. The method of claim1 wherein:said at least one CDMA spreading code used in said at leastone said neighboring cell, of which said base station informs saidmobile station in said informing step comprises a spreading code used ona pilot channel of a respective neighboring cell.
 5. The method of claim1, wherein:in conducting said informing, said base station transmits tosaid mobile station on a control channel.
 6. The method of claim 1,wherein:in conducting said informing, said base station informs saidmobile station in regard to less than all spreading codes in use by saidat least one neighboring cell, by including at least one more likely tocorrelate with multiple-access interference, and by excluding at leastone less likely to correlate with multiple-access interference, asexperienced by said mobile station.
 7. The method of claim 6,wherein:said including and excluding is practiced at least in part basedon relative power levels of transmissions by the at least oneneighboring base station.
 8. The method of claim 6, further comprising:abase station controller in control of said base station determining forsaid base station which CDMA spreading codes said base station willinform said mobile station of in said informing step.
 9. The method ofclaim 1, further comprising:practicing of said informing step beinginitiated by said mobile station requesting said base station to soinform said mobile station.
 10. The method of claim 1, wherein:said basestation is a presently dedicated base station as to at least anotherrespective one of said mobile stations; and said informing step ispracticed by said base station in regard to less than all of said mobilestations for which said base station is a presently dedicated basestation, depending on relative likelihood of experiencingmultiple-access interference.
 11. A mobile station useable foreliminating multiple-access interference when communicating in a CDMAcellular radio system having a plurality of cells each comprising atleast one base station arranged for communicating by CDMA with mobilestations present in a geographic area covered by the respective cell,when said base station is performing as a dedicated base stationrelative to at least one mobile station, including said base station,and while bordered by at least another, neighboring said cell having arespective neighboring base station, said mobile stationcomprising:receiver means for causing said mobile station to becomeinformed by said base station of at least one CDMA spreading code usedin said one cell and at least one CDMA spreading code used in at leastone said neighboring cell, and for detecting components of a desiredsignal and a signal to be measured, respectively transmitted by saidbase station and said at least one neighboring base station, onrespective channels, using respective CDMA spreading codes; measuringmeans arranged for measuring the code phase and power level of arespective channel of each of said one cell and said at least oneneighboring cell, using respective CDMA spreading codes as made known tosaid mobile station by said practicing of said informing; and detectingby eliminating means for detecting, from signal components therebyreceived, said desired signal, by eliminating interference from saidsignal components as received by said mobile station, based on saidmeasuring.
 12. The mobile station of claim 11, wherein said detecting byeliminating means is arranged for:estimating interference caused by atleast one said respective channel of said at least one neighboring cellas made known to said mobile station by said practicing of saidinforming, by:calculating a cross-correlation between components of saiddesired signal as detected from respective transmissions of said onebase station and said at least one neighboring base station, andweighting said cross-correlation with respective power levels obtainedin said measuring; and for subtracting said mobile station interferencethereby estimated, from the signal components received, to obtain thedesired signal.
 13. The mobile station of claim 11, furtherincluding:storage means for storing information about the CDMA spreadingcodes in use on connectors of said at least one neighboring base stationwhich are capable of interfering with reception by said mobile stationof said desired signal; said mobile station being arranged for usingsaid information as stored by said storage means, in practicing saideliminating.